We believe that vasoreactivity with acute challenge of potent IV vasodilators [Prostacyclin or Iloprost] will predict long term oral vasodilator therapy success in patients with pulmonary hypertension. We wish to establish to what extent pulmonary hypertension is amenable to chronic vasodilator therapy and to determine which therapy is effective under which conditions. We are interested in additing any new knowledge to our understanding of the etiology of pulmonary hypertension.